Certain devices comprise or make use of a surface relief micro-structure. Examples of devices which require an accurate production of a surface micro-structure include holograms, optically variable devices (OVDs), Fresnel lenses and e-beam and lithography originations. For the purposes of repeated production of a particular device, the surface relief micro-structure must be copied many times.
One known way of copying a surface relief micro-structure is using a standard press having a head to which an inverse relief of the micro-structure to be copied is attached. The press is operated such that the head is pressed into a heated thermoplastic, thereby forming the required relief in the thermoplastic. When the thermoplastic cools and sets, the relief is set within the plastic.
There are various problems with the thermoplastic method. Firstly, there is a limitation on the accuracy of the relief which can be set and this accuracy is often insufficient for the quality of product required. Secondly, there are limitations on the depth, feature aspect ratios and complexity of relief that can be produced. Thirdly, the high pressure with which the inverse relief is pressed into the thermoplastic causes damage to the inverse relief over multiple copies, thus necessitating replacement of the inverse relief more often than is desirable when mass-producing a particular product. Fourthly, the action of bringing a press down onto the thermoplastic can result in the entrapment of air bubbles which has an adverse affect on the quality of the relief produced. Equally, removal of the head can cause a problem because the head is liable to stick to the thermoplastic and thereby damage the produced relief.
It would be desirable to provide an improved machine head and method for producing a surface relief on a substrate.